Lecture: 57 pressure, blood flow, compliance and resistance

Lecture -57
Pressure, Blood Flow, Compliance and Resistance
Dr Shamshad
Majamaah University
KSA

Learning Objectives
1. Describe the interrelationship of pressure, flow & resistance.
2. Describe the laminar blood flow & causes of its turbulence
3. Define vascular distensibility & compliance. And identify the
difference b/w arterial & venous Compliance.
4. Identify the standard units of Blood Pressure measurements.
5. Identify the units for measurements of Resistance.
6. Relate these factors to control of Arterial Pressure.

Revision
Types of circulation : Pulmonary & systemic circulation
Functions of circulation :[Heart &blood vessels]
 Supplies nutrients according to need of the tissues.
 Removes wastes products of tissue metabolism
 Body defense mechanisms.
 Control blood flow to the skin & limbs.
 Ensuring one-way blood flow: with help of valves
 Regulating blood supply according to the need of the body
increase in exercise & pregnancy
 Generating blood pressure.

Types of blood vessels based on functions
1. Windkessel /Distensible vessels
3. Exchange vessels
4. Capacitance vessels
2. Resistance vessels
5. Shunts vessels

Hemodynamic of blood flow
 Pressure
 Flow of blood in vessels
 Resistance to flow
If blood pressure increases, blood flow increases;
If peripheral resistance increases, blood flow decreases.

Blood flow(ml/min ) : The volume of blood flowing through a
vessel/organ/the entire circulation in a given period .
[Entire circulation =cardiac output, 5000ml].
Blood pressure: (mm Hg):The force per unit area exerted by the
blood against a vessel wall.
Resistance: A measure of the friction b/w blood & the vessel wall,
Sources:1)blood viscosity, 2) vessel length, 3) vessel diameter.
Diameter has greatest effect on resistance of a particular vessel -
resistance drops exponentially as the radius increases
TPR Total peripheral resistance :Resistance throughout the entire
systemic circulation.

1.Diameter 2.Cross sectional area
3.Blood pressure 4.Velocity

Distribution of total blood volume in different parts of
circulation
Systemic
circulation=84%
Heart &lungs
=16%
Veins 64% Heart 7%
Arteries 13% Pulmonary
vessels
9%
Arterioles
and
capillaries
7%
Total blood volume =5000ml (100%)

Flow –Cross sectional area relationship
1. Increase in total
cross-sectional area of
blood vessel 
Decreases the linear
mean velocity of blood
flow.
2. If the diameter of the
blood vessel is narrowed
,keeping the rate of flow
constant the velocity of
flow increases.
Significance:-

Cross-sectional areas of the
veins are much larger than
those of the arteries, (about
four times).
Hence large blood storage
capacity of the venous
system in comparison with
the arterial system.
Vessel Cross-Sectional Area
(cm2)
Aorta 2.5
Small arteries 20
Arterioles 40
Capillaries 2500
Venules 250
Small veins 80
Venae cavae 8

Factors that determine Blood flow in blood vessel :
1] Pr gradient: Push the blood through the vessel .
Pr difference of the blood b/w two ends of vessel along the
vessel .
2] Vascular resistance :Resistance by friction b/w the flowing
blood & the intravascular endothelium all along the inside of the
vessel.
Calculated by :Ohm’s law:-

Laminar /streamline blood flow
Defn: Blood flows at steady rate through long smooth blood
vessels with each blood layer remaining at same distance from
the vessel wall.
Creates parabolic velocity.
Cause: Adherence of fluid molecules at wall
 moves very slow as compared to the central layer.
Laminar flow

Turbulent Flow
Depends on:
 Velocity
 diameter of vessels
 density of blood
 & viscosity of blood flow
 Reynold’s number: >2000 causes turbulence in streamline flow.

Blood Pressure
Defn : Force exerted by the blood against the any unit area of
the vessel wall.
Unit: mm Hg.

Resistance to flow
Defn: Obstruction to blood flow.
Measured indirectly
Determined from pressure & flow measurements.
Systemic circulatory system:
 Called Total peripheral resistance (TPR)
Pulmonary system
 Called total pulmonary resistance =1/7th of TPR

Conductance
Conductance : 1/resistance
Unit : ml/sec/mmHg
Conductance=(vessel diameter)4
Conductance helps to calculate the total resistance of parallel
vessels.

Vessel Diameter & Blood Flow :Changes in Resistance

Adding Resistance in Series & Parallel
R=1/C

Significance: parallel circuit
 In brain ,kidneys, GIT, muscles ,skin & coronary circulations
vessels
 Each vessels contributes to overall conductance of systemic
circulation.
 Blood flow through each tissue is fraction of CO.
 Determined by resistance for blood flow & pr gradient.
Significance: if one kidney is removed
CO decreases & Increase in TPR.

Effect of Viscosity on Resistance & Blood Flow
Significance : As in polycythemia
Increase viscosity increases resistance & decreases flow

Vascular Distensibility
Defn: Character of the vessels that allows to accommodate the
pulsatile output of the heart & to average out the pressure
pulsations.
 Function: Provides smooth & continuous flow of blood
through very small blood vessels of tissues.
 capacitance vessels :Veins
 Distensibility= vol/ pr*original vol.
 Artery less distensible as walls more stronger.
 larger distensibility of veins allows to act as blood reservoirs.

Vascular Compliance/Capacitance
Defn: The quantity of blood that can be stored in a given portion of
the vasculature for each mmHg rise in pr.
Vascular compliance = Increase in volume/Increase in pressure
Compliance = distensibility x volume.
 Effect of Sympathetic NS:
 Reflex Increase vascular tone through systemic circulation can
cause large vol of blood to shift into heart which increase heart
pumping.
 In hemorrhage : SNS tone play important role.

Difference b/w arterial & venous compliance.
Arteries
• thick walls
• Elastic & muscular wall
• NE act on smooth muscle α1
receptors leads to
contraction
• & decrease compliance
Veins
• Thin walls
• More compliant
• Capacitance vessels
• Stores >60% of blood
• distend to hold excess
blood
Ex. during a transfusion

Factors that control ABP
 Arterial pr pulsation: Aorta top pulsation is120 & low 80 mmHg
Hence 120-80=40 mmHg = pulse pressure(PP)
Factors affecting are
 Stroke volume
 Compliance /total distensibility
 Viscosity of blood
 Velocity of blood flow
 Character of ejection of blood from heart during systole

Lecture: 57 pressure, blood flow, compliance and resistance

Lecture: 57 pressure, blood flow, compliance and resistance

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